1. Field of the Invention
The invention relates to low shear-thinning polyvinyl acetals, processes for the preparation of low shear-thinning polyvinyl acetals, and the use thereof.
2. Description of the Related Art
The preparation of polyvinyl acetals which are obtained from the corresponding polyvinyl alcohols by polymer-analogous reaction with the corresponding aldehydes has been known since 1924, and a multiplicity of aldehydes have subsequently been used for the preparation of the corresponding polyvinyl acetals. Polyvinyl acetals are prepared in a 3-stage process (polyvinyl acetate→polyvinyl alcohol→polyvinyl acetal), products which also contain vinyl alcohol and vinyl acetate units in addition to vinyl acetal groups resulting. Polyvinyl formal, polyvinyl acetacetal and polyvinyl butyral (PVB) are of particular commercial importance.
Inter alia because of their good pigment binding power, polyvinyl butyrals are also used as binders in coatings and especially in printing inks. In this Application, there is the requirement that the organic solutions of the polyvinyl acetals have as constant a solution viscosity as possible even at high shear load, i.e. also in high-speed printing presses. Newtonian behaviour is required, regardless of the applied shear rate.
EP 535643 A1 relates to a process for the preparation of polyvinyl acetals having improved melt viscosity by means of acetalation of vinyl alcohol, copolymers with a proportion of polyethylenically unsaturated comonomers. For the acetalation, polyvinyl alcohol and aldehyde are initially introduced and the acid is metered in continuously. JP-A 2001-288215 discloses a process for the preparation of polyvinyl acetals resins, in which polyvinyl alcohol and aldehyde are each partly initially introduced, and the remaining proportions are each metered in portions after cooling of the reaction mixture. In JP-A 2000-159831, for the preparation of polyvinyl acetal, the main proportion of the polyvinyl alcohol is initially introduced, acid catalyst and aldehyde are added and the remainder of polyvinyl alcohol is metered in. In the process of JP-A 10-158328, the acetalation is optimized by means of addition of already acetalated polyvinyl alcohol. WO 03/066690 relates to a process for the preparation of polyvinyl acetals having low metal contents, the acetalation of polyvinyl alcohol being effected in two reactors connected in series.
J. IMAG. SCI. TECHNOL. 43, 535 (1999) investigates polyvinyl acetal solutions in methyl ethyl ketone whose solution viscosity changes drastically in a shear rate range from 1 to 100 s−1. DE-A 2365005 describes a process for the preparation of polyvinyl acetals wherein the acetalation is carried out in two temperature ranges and the proportion of aldehyde is in each case not metered in continuously but is added intermittently. EP 594026 A2 describes a process for the preparation of polyvinyl acetal dispersions wherein the aldehyde portion is metered in continuously over several hours.